Modern communication systems are typically partitioned into layers. An example of such a partitioning is the seven-layer Open Systems Interconnection model (OSI model), devised by the International Organization for Standardization. The OSI model categorizes the portion of the communications system that is closest to the physical medium as the physical layer (PHY), which is often referred to as the lowest of the seven layers. Therefore, the other six layers in the OSI model can be referred to as higher layers relative to the PHY. These abstract layers above the PHY perform various functions, for example, authentication, bridging, routing, and application services. When a network device sends a communication to another network device, the communication passes down through the layers of the protocol stack of the sending device and up through layers of the protocol stack of the receiving device. Each given layer communicates with either the next layer above or the next layer below that given layer. The protocol at each given layer corresponds to that set of rules followed in order provide the services of that given layer.
In practice, implementations of communication systems do not always precisely partition into those layers specified by models such as the OSI model. In addition, the dividing line between the PHY and the higher layers may differ from implementation to implementation and, for some implementations, may be blurry, where operations may be grouped either with the PHY or with the higher layers. Nonetheless, irrespective of where the dividing line lies, the layers can be partitioned into at least two segments: the PHY and the higher layers.
Data problems can be detected at any one or more of the layers. For example, faults in the physical media can corrupt data units. If a data unit is erred, the protocol layer processing the data unit may discard the data unit or transfer the data unit to the next layer in the protocol stack, in effect propagating the error to a higher layer. Eventually, if correction of the data unit does not occur, either by retransmission or by a form of error correction, the corrupt data can degrade the quality of the application layer. For example, a television receiving video information over a data network may experience a display anomaly as a result of the erred data.
Controls are typically applied at each of the layers to minimize errors and optimize the performance of the layer for a given communication link. However, the precise effect of the controls applied at the PHY upon the quality of the communication link at the higher layers is, for the most part, unknown. In practice, adjusting the PHY without regard to the higher layer may be of little or no consequence to higher-layer link quality.